Heart disease is the leading cause of death in the United States. Although current therapies have reduced mortality, patients surviving the initial stages of cardiac injury suffer more prevalent chronic disruptions of cardiac function, resulting in the necessity for continuous medical care. Establishing therapies that can reduce the long-term effects of cardiac injury, such as fibrosis, could facilitate cardiac healing and improve function. The goal of this proposal is to provide valuable insight into cardiac fibroblast development and function. The epicardium contributes coronary vascular smooth muscle cells, and cardiac fibroblasts to the developing heart, and data suggests the adult epicardium maintains this potential. Studying the epicardium and its derivatives may aid in understanding the cardiac healing process and provide potential therapeutic targets to prevent long-term consequences of cardiac injury. Platelet derived growth factor receptors (PDGFR) are receptor tyrosine kinases that provide signals important for fibroblast migration and proliferation, and are expressed in the epicardium and cardiac fibroblasts. We propose to investigate the role of PDGFRa in the development and function of cardiac fibroblasts through the following specific aims: First, 1 will confirm PDGFRa expression in cardiac fibroblasts and then determine whether PDGFRa is required for the embryonic development of cardiac fibroblasts by analyzing a tissue-specific deletion of the receptor in the epicardium. Subsequently, I will investigate all stages of epicardial development for proliferation, apoptosis, EMT, and migration defects using histology, flow cytometry, and immunohistochemistry. 1 will also analyze if fibroblast differentiation is affected using the same techniques. Second, 1 will investigate PDGFRa function in adult cardiac fibroblasts in vitro and in vivo. This will be done by conditionally ablating expression in cultured fibroblasts and measuring their activation in response to stimulation. Subsequently, 1 will use experimentally induced fibrosis with mice lacking PDGFRa in cardiac fibroblasts and analyze the hearts by standard histological and biochemical techniques for changes in the fibrotic response. Lay summary: To improve long-term outcomes after heart attacks, such as fibrosis, we need to understand both the development and function of the reactive cells. Here, we propose to study some of the signals implicated in heart fibrosis. Our results may uncover new methods to prevent this detrimental side effect of heart attacks from occurring.